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31 Exception Path Sample Application
32 =================================
34 The Exception Path sample application is a simple example that demonstrates the use of the DPDK
35 to set up an exception path for packets to go through the Linux* kernel.
36 This is done by using virtual TAP network interfaces.
37 These can be read from and written to by the DPDK application and
38 appear to the kernel as a standard network interface.
43 The application creates two threads for each NIC port being used.
44 One thread reads from the port and writes the data unmodified to a thread-specific TAP interface.
45 The second thread reads from a TAP interface and writes the data unmodified to the NIC port.
47 The packet flow through the exception path application is as shown in the following figure.
49 .. _figure_exception_path_example:
51 .. figure:: img/exception_path_example.*
56 To make throughput measurements, kernel bridges must be setup to forward data between the bridges appropriately.
58 Compiling the Application
59 -------------------------
61 To compile the sample application see :doc:`compiling`.
63 The application is located in the ``exception_path`` sub-directory.
65 Running the Application
66 -----------------------
68 The application requires a number of command line options:
70 .. code-block:: console
72 .build/exception_path [EAL options] -- -p PORTMASK -i IN_CORES -o OUT_CORES
76 * -p PORTMASK: A hex bitmask of ports to use
78 * -i IN_CORES: A hex bitmask of cores which read from NIC
80 * -o OUT_CORES: A hex bitmask of cores which write to NIC
82 Refer to the *DPDK Getting Started Guide* for general information on running applications
83 and the Environment Abstraction Layer (EAL) options.
85 The number of bits set in each bitmask must be the same.
86 The coremask -c or the corelist -l parameter of the EAL options should include IN_CORES and OUT_CORES.
87 The same bit must not be set in IN_CORES and OUT_CORES.
88 The affinities between ports and cores are set beginning with the least significant bit of each mask, that is,
89 the port represented by the lowest bit in PORTMASK is read from by the core represented by the lowest bit in IN_CORES,
90 and written to by the core represented by the lowest bit in OUT_CORES.
92 For example to run the application with two ports and four cores:
94 .. code-block:: console
96 ./build/exception_path -l 0-3 -n 4 -- -p 3 -i 3 -o c
101 While the application is running, statistics on packets sent and
102 received can be displayed by sending the SIGUSR1 signal to the application from another terminal:
104 .. code-block:: console
106 killall -USR1 exception_path
108 The statistics can be reset by sending a SIGUSR2 signal in a similar way.
113 The following sections provide some explanation of the code.
118 Setup of the mbuf pool, driver and queues is similar to the setup done in the :ref:`l2_fwd_app_real_and_virtual`.
119 In addition, the TAP interfaces must also be created.
120 A TAP interface is created for each lcore that is being used.
121 The code for creating the TAP interface is as follows:
126 * Create a tap network interface, or use existing one with same name.
127 * If name[0]='\0' then a name is automatically assigned and returned in name.
130 static int tap_create(char *name)
135 fd = open("/dev/net/tun", O_RDWR);
139 memset(&ifr, 0, sizeof(ifr));
141 /* TAP device without packet information */
143 ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
145 rte_snprinf(ifr.ifr_name, IFNAMSIZ, name);
147 ret = ioctl(fd, TUNSETIFF, (void *) &ifr);
156 snprintf(name, IFNAMSIZ, ifr.ifr_name);
161 The other step in the initialization process that is unique to this sample application
162 is the association of each port with two cores:
164 * One core to read from the port and write to a TAP interface
166 * A second core to read from a TAP interface and write to the port
168 This is done using an array called port_ids[], which is indexed by the lcore IDs.
169 The population of this array is shown below:
176 RTE_LCORE_FOREACH(i) {
177 if (input_cores_mask & (1ULL << i)) {
178 /* Skip ports that are not enabled */
179 while ((ports_mask & (1 << rx_port)) == 0) {
181 if (rx_port > (sizeof(ports_mask) * 8))
182 goto fail; /* not enough ports */
184 port_ids[i] = rx_port++;
185 } else if (output_cores_mask & (1ULL << i)) {
186 /* Skip ports that are not enabled */
187 while ((ports_mask & (1 << tx_port)) == 0) {
189 if (tx_port > (sizeof(ports_mask) * 8))
190 goto fail; /* not enough ports */
192 port_ids[i] = tx_port++;
199 After the initialization steps are complete, the main_loop() function is run on each lcore.
200 This function first checks the lcore_id against the user provided input_cores_mask and output_cores_mask to see
201 if this core is reading from or writing to a TAP interface.
203 For the case that reads from a NIC port, the packet reception is the same as in the L2 Forwarding sample application
204 (see :ref:`l2_fwd_app_rx_tx_packets`).
205 The packet transmission is done by calling write() with the file descriptor of the appropriate TAP interface
206 and then explicitly freeing the mbuf back to the pool.
210 /* Loop forever reading from NIC and writing to tap */
213 struct rte_mbuf *pkts_burst[PKT_BURST_SZ];
216 const unsigned nb_rx = rte_eth_rx_burst(port_ids[lcore_id], 0, pkts_burst, PKT_BURST_SZ);
218 lcore_stats[lcore_id].rx += nb_rx;
220 for (i = 0; likely(i < nb_rx); i++) {
221 struct rte_mbuf *m = pkts_burst[i];
222 int ret = write(tap_fd, rte_pktmbuf_mtod(m, void*),
224 rte_pktmbuf_data_len(m));
227 lcore_stats[lcore_id].dropped++;
229 lcore_stats[lcore_id].tx++;
233 For the other case that reads from a TAP interface and writes to a NIC port,
234 packets are retrieved by doing a read() from the file descriptor of the appropriate TAP interface.
235 This fills in the data into the mbuf, then other fields are set manually.
236 The packet can then be transmitted as normal.
240 /* Loop forever reading from tap and writing to NIC */
244 struct rte_mbuf *m = rte_pktmbuf_alloc(pktmbuf_pool);
249 ret = read(tap_fd, m->pkt.data, MAX_PACKET_SZ); lcore_stats[lcore_id].rx++;
250 if (unlikely(ret < 0)) {
251 FATAL_ERROR("Reading from %s interface failed", tap_name);
256 m->pkt.data_len = (uint16_t)ret;
258 ret = rte_eth_tx_burst(port_ids[lcore_id], 0, &m, 1);
259 if (unlikely(ret < 1)) {
261 lcore_stats[lcore_id].dropped++;
264 lcore_stats[lcore_id].tx++;
268 To set up loops for measuring throughput, TAP interfaces can be connected using bridging.
269 The steps to do this are described in the section that follows.
271 Managing TAP Interfaces and Bridges
272 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
274 The Exception Path sample application creates TAP interfaces with names of the format tap_dpdk_nn,
275 where nn is the lcore ID. These TAP interfaces need to be configured for use:
277 .. code-block:: console
279 ifconfig tap_dpdk_00 up
281 To set up a bridge between two interfaces so that packets sent to one interface can be read from another,
284 .. code-block:: console
287 brctl addif br0 tap_dpdk_00
288 brctl addif br0 tap_dpdk_03
291 The TAP interfaces created by this application exist only when the application is running,
292 so the steps above need to be repeated each time the application is run.
293 To avoid this, persistent TAP interfaces can be created using openvpn:
295 .. code-block:: console
297 openvpn --mktun --dev tap_dpdk_00
299 If this method is used, then the steps above have to be done only once and
300 the same TAP interfaces can be reused each time the application is run.
301 To remove bridges and persistent TAP interfaces, the following commands are used:
303 .. code-block:: console
307 openvpn --rmtun --dev tap_dpdk_00